In some 'Harry Potter turns evil' fanfictions, the death of a loved one, like Sirius, could be the tipping point. Grief and anger could consume Harry. He might think that the light side of magic has failed him. So, he starts exploring the dark arts more deeply. He could also form alliances with Death Eaters who promise him power and the ability to change the wizarding world according to his new, darker vision. As he delves deeper into the dark side, his personality changes completely, becoming cold and calculating, unlike the kind Harry we know from the original books.
Well, perhaps in this fanfiction, Harry is constantly bullied at Hogwarts in a much more extreme way. This continuous abuse could warp his sense of right and wrong, making him seek revenge on those who have wronged him. He might start using dark magic to get back at his tormentors and gradually slide into evil. He could also lose faith in Dumbledore's leadership and view of the world, which could be a major factor in his turn to the dark side.
One possible plotline could be that Harry discovers a dark secret about his family and feels betrayed by the wizarding world. This could lead him to side with the dark forces. Another could be that he is influenced by a powerful dark wizard who manipulates his mind. Maybe he becomes envious of the power others hold and decides to take it by any means, turning evil in the process.
Maybe Harry discovers some dark family secrets that make him question his place on the 'good' side. He could find out that his ancestors had strong ties to dark magic, and this knowledge could slowly corrupt him. He might start to experiment with dark spells, thinking that he can control them and use them for what he believes is the greater good, but gradually slips towards the darkside.
One possible plotline could be that Voldemort rescues Harry because he sees potential in him to be a powerful ally in his quest for total domination. Harry, being evil, might have already started to dabble in the Dark Arts on his own and Voldemort wants to harness that. Another plotline could be that Voldemort saves Harry from some other threat, like the Ministry of Magic which has turned against Harry for his evil deeds. This could lead to a strange alliance between them as they both have a common enemy in the Ministry.
One possible plotline could be that Naruto is constantly shunned and mistreated by the village despite his efforts to protect it. This continuous rejection could gradually turn his heart cold and make him seek revenge. He might start using his powerful jutsu against those who once wronged him, like attacking the Hidden Leaf's leadership.
One possible plotline could be that Lily, now as a girl, has to navigate the wizarding world all over again. She might face different challenges and opportunities than she did before. For example, she could form new friendships or rivalries at Hogwarts.
One possible plotline could be Harry struggling to control his new veela - like charm powers and accidentally causing chaos at Hogwarts. For example, he might make his classmates act strangely around him, much to the confusion of his friends.
A great plotline could be that Harry, now a girl, has to navigate the world of wizarding fashion and etiquette which she previously ignored. She might be forced to attend more social events at Hogwarts, like balls or tea parties, where she has to learn proper behavior. And she could also use her new identity to her advantage when it comes to spying on the Death Eaters. They might underestimate her because she's a girl, but she could outwit them.
In some 'Harry Potter turns into Hermione fanfiction', the plot may involve Harry adjusting to Hermione's physical appearance as well. He might be shocked at first but then realizes the advantages it gives him. For example, he could use Hermione's reputation for being a bookworm to his advantage when it comes to researching dark magic or hidden secrets in the library. And he may also face some difficulties when it comes to Hermione's friendships with other girls, as he has to act like her and keep those relationships intact.
One possible plotline could be that Twilight is influenced by a powerful dark magic artifact. Maybe she discovers an ancient book that slowly corrupts her mind, making her turn against her friends. As she turns evil, she might start to use her powerful magic to take over Ponyville, believing that she can create a 'better' world under her rule.
One possible plotline could be that the Veela is drawn to Harry's new - found evil side, perhaps seeing power in it. And Harry uses the Veela's charm and abilities for his evil plans, like luring enemies or getting information. Another could be that the Veela tries to redeem Harry from his evil ways, creating a sort of internal struggle within Harry between his evil intentions and the Veela's positive influence.
One possible plotline could be Harry discovers his latent archmage powers early on and uses them to quickly defeat Voldemort. Another could be that he has to go on a journey to learn how to fully control his archmage abilities while still dealing with the normal Hogwarts stuff like classes and friendships.